1. Field of the Invention
The present invention relates to an electric power steering apparatus using a coated pulley.
2. Description of the Related Art
In a steering system of a vehicle, a so-called power steering apparatus for assisting steering operation using an external power source has been widely employed. Conventionally, a vane type hydraulic pump has been used as the power source for the power steering apparatus, so that this hydraulic pump is mainly driven by the engine. However, this type of power steering apparatus has a large power loss (e.g., several to several tens horsepower at a maximum load) of the engine because of driving the hydraulic pump at all times, so that it is hard to apply the power steering apparatus to a light weight and small engine displacement vehicles. Even a comparatively large engine displacement vehicle comes to have the inevitable demerit that its fuel consumption rate is lowered enough not to be negligible.
Accordingly, in order to solve the problems, an electric power steering (EPS) apparatus utilizing an electric motor as a power source has attracted the attention over the recent years. Since the electric power steering apparatus uses a battery mounted in the vehicle as a power source for the electric motor, there is no direct drive loss of the engine. In addition, the electric motor is started only when assisting the steering operation, so that the decrease in the fuel consumption rate is restrained and the electronic control can be extremely easily performed.
In the meantime, as a steering gear for a passenger car, a rack-and-pinion type steering gear is mainly employed recently, since the rack-and-pinion type gear has high rigidity performance and is low in weight. As the electric power steering apparatuses for the rack-and-pinion steering gear, there are provided a column assist type in which the electric motor is installed to a side of a column to drive a steering shaft or a pinion itself, or a ball screw rack assist type in which a rack shaft is driven by an electrically-driven ball screw mechanism. In the ball screw rack assist type electric power steering apparatus, the assist force is not exerted on an engaging surface between the pinion and the rack, so that a contact surface pressure between both the members, which causes abrasion and deformation, is relatively small.
FIG. 1 is a perspective view showing an electric power steering apparatus according to a prior art.
Referring to FIG. 1, a steering column 1 serves to rotatably support an upper pinion shaft 3. A steering wheel 5 is mounted to an upper end of the upper pinion shaft 3, and a lower pinion shaft 9 is coupled to a lower end of the upper pinion shaft 3 through a universal joint 7. A steering gear 11 including a rack-and-pinion mechanism or a power assist mechanism is connected to a lower end of the lower pinion shaft 9.
As shown in the figure, a column cover 13 is provided to cover the steering column 1. Also, tie rods 15 are connected to right and left ends of the steering gear 11.
FIG. 2 is a longitudinal sectional view of an electric power steering gear according to the prior art.
Referring to FIG. 2, a rack-and-pinion housing 21 keeps a rack shaft 23 and a pinion (not shown) of the rack-and-pinion mechanism to be accommodated therein. In the rack shaft 23, a rack 25 meshed with the pinion is provided in the left side of the power steering gear 11. In addition, spherical joints 27 supporting hingedly the tie rods 15 are fixed to the right and left ends of the power steering gear 11.
A belt housing 31 bolted to a right end of the rack-and-pinion housing 21 and a ball screw housing 33 bolted to the belt housing 31 and defining an electric steering gear case together with the rack-and-pinion housing 21 and the belt housing 31, define an outer shell of the power assist mechanism.
An electric motor 35 is attached to a lower side of the belt housing 31, and the gear housing 31 has a power transmission mechanism accommodated therein, i.e., a driving pulley 37 fixed to a shaft 36 of the electric motor 35, a hollow driven pulley 39 coupled to a ball nut 51 and a belt 41 wound on the driving pulley 37 and the driven pulley 39.
In the ball screw housing 33, the ball nut 51 is maintained to be rotatable through a double-row angular ball bearing 53.
Also, the rack shaft 23 is formed with a male thread groove in the shape of a ball screw, while the ball nut 51 is formed with a female thread groove in the shape of a ball screw. A plurality of steel balls 65, as circulation balls, are interposed between the male thread groove and the female thread groove. Moreover, the ball nut 51 is mounted with a circulation piece (not shown) for causing the steel balls 65 to circulate.
In addition, a female spline is formed in an end of the driven pulley 39 corresponding to the ball nut 51, while a male spline engaged with the female spline is formed on the ball nut 51. Accordingly, the driven pulley 39 and the ball nut 51 are integrally rotated with assembled.
The operation of the electric power steering apparatus will be described with reference to FIG. 1 and FIG. 2 as follows.
Once a driver turns the steering wheel 5, the rotational force thereof is transferred to the steering gear 11 via the upper steering shaft 3 and the lower steering shaft 9. Since the steering gear 11 includes therein the rack-and-pinion mechanism for converting a rotational motion into a rectilinear motion, the rack shaft 23 moves in any one of the right and left directions. Therefore, a steering angle of road wheels is changed through the right and left tie rods 15, and thus, the steering operation is performed.
Simultaneously, in the power assist mechanism, the electric motor 35 rotates at a predetermined rotational torque in a forward or reverse direction based on an output of a steering torque sensor (not shown). The rotation of the electric motor is reduced and transferred to the ball nut 51 via the driving pulley 37 and the driven pulley 39. The ball nut 51 is spline-coupled to and cooperated with the driven pulley 39. When the ball nut 51 is rotated, a thrust force is exerted on the male thread groove of the rack shaft 23 through the steel balls 65 provided in the female thread groove, so that a steering assist operation is performed.
In the operation of the electric motor, the driving and driven pulleys 37 and 39 are in surface contact with the belt 41 to make the belt rotate in the caterpillar manner. It is true that the driving and driven pulleys 37 and 39 are continuously in surface contact with a surface of the belt 41, thus wearing and being damaged. Also, the driving and driven pulleys 37 and 39 can be manufactured by machining general carbon steel having a relatively good processability. Due to the excellent processability, the general carbon steel has been used widely for manufacturing mechanical parts requiring a dimensional precision.
However, in the parts such as the driving pulley and the driven pulley employed in the electric power steering apparatus according to the prior art, since a gap between teeth thereof is small, it is difficult to lower a surface roughness of the pulleys to a certain level when the pulleys are manufactured by machining general carbon steel. Consequently, due to the above problem, the belt that is a counterpart of the driving and driven pulleys wears, so that the durability of the belt can be reduced. In particular, due to hardening of the surface of the belt in the early stage of operation of the apparatus at a low temperature, there is a problem in that an abnormal friction is generated between the surfaces of the pulley and the belt, thus generating a noise and slip phenomenon.